This invention relates generally to ignition lead assemblies and, more particularly, to methods and apparatus for connecting ignition lead assemblies within gas turbine engines.
Gas turbine engines typically include ignition systems to provide ignition to a fuel and air mixture within the gas turbine engine. The gas turbine engine ignition systems include lead assemblies connected to engine exciters and engine igniters. Specifically, the lead assembly connectors are connected to an igniter cable housed within a flexible conduit. The igniter cable includes a stranded center conductor encased within electrical insulation which is permeable to varying degrees. Each connector housing contains terminal dielectrics sealed with silicone grommets. The connectors are sealed so that air trapped in each connector has a pressure equal to that of atmospheric pressure.
In use, and at altitude, because the engine lead assembly connectors are not hermetically sealed, the air initially trapped within the sealed connectors slowly escapes from the connectors through the permeable ignition cable electrical insulation. At ground-level, air slowly seeps into the connectors through the ignition cable electrical insulation. Because air seeps into the connectors at approximately the same rate as air escapes from the connectors, the connectors on engines that operate more frequently or for longer durations at altitude are subjected to lower average air pressures in comparison to connectors on engines that operate less frequently or for shorter durations at altitude.
Operating the ignition system with reduced air pressure in the sealed ignition lead to engine exciter connection and the sealed ignition lead to engine igniter connection may cause partial electrical discharges, known as corona. Over time, continued exposure to corona may lead to damage of terminal dielectrics housed within the connectors. To minimize the effects of corona, at least some known engine ignition systems include molding or corona suppressants to reduce the amount of air trapped within the ignition system. Such molding or corona suppressants may be expensive, add complexity to the ignition system, and are difficult to inspect for conformance to quality requirements. Other ignition systems include connectors using various configurations and surface shapes to increase the ignition system""s tolerance of corona, and often eliminate sharp edges on sub-components of the connectors to reduce a strength of local electric fields which can lead to corona.
In an exemplary embodiment, an ignition lead assembly includes a seal sub-assembly that prevents contamination from entering the ignition lead assembly and prevents complete depressurization of the ignition lead assembly, thus facilitating a reduction in the formation of corona within ignition lead assembly connectors. In a second embodiment, the seal assembly permits the ignition lead assembly to be pressurized to facilitate a reduction in the formation of corona within ignition lead assembly connectors. The ignition lead assembly includes an ignition cable housed within a conduit and attached at each end to connectors. The ignition cable includes a plurality of wires encased within permeable electrical insulation. The conduit includes an air-cooled portion and a non-air-cooled portion connected together with a coupling assembly. The seal sub-assembly includes a housing, a seal, a retainer, and a biasing mechanism, and prevents a loss of air pressure from the non-air-cooled portion of the ignition lead assembly.
In use, cooling air is channeled into the conduit air-cooled portion at a pressure only slightly above that of engine core cavity ambient air pressure, thus creating a negative pressure differential between the inside and outside of the ignition cable, causing air to escape radially outward from the inside of the ignition cable through the permeable electrical insulation.
The seal sub-assembly traps air in the conduit non-air-cooled portion of the lead assembly preventing a loss of air pressure. In the second embodiment, an external air source simultaneously directs pressurized airflow into the non-air-cooled portion of the lead assembly. In both embodiments, a pressure differential between the inside and outside of the ignition cable within the non-air-cooled portion is positive causing air to seep into the ignition cable inside through the ignition cable electrical insulation. Because the pressure differential across the ignition cable within the conduit non-air-cooled portion is greater than the pressure differential across the ignition cable within the conduit air-cooled portion, a pressure balance occurs across the ignition cable and air trapped within the connectors is pressurized. The pressurized air within the connectors facilitates a reduction in the formation of potentially damaging corona, thus extending a useful life of the ignition lead assembly. As a result, the ignition lead assembly facilitates reducing potentially damaging arcing within the connectors.